(Mishnah): If a person's field was planted with onions and he wanted to plant there rows of gourds - R. Yishmael says that he should uproot two rows (of onions, which is a total width of 8 Amos) and then plant one (4 Amah) row of gourds (inside the middle of the cleared space); then leave two rows of onions standing, and again uproot two rows (of onions) and plants a row (of gourds in the cleared space).

(R. Akiva): He should uproot two rows of onions and plant two rows of gourds (in their place); then he leave two rows of onions standing and then uproot two rows and plant two rows of gourds (in their place).

(Chachamim): If there aren't 12 Amos between one row and the next, he may not retain the onions in between them.

(Gemara) (Kahana): According to R. Yishmael, the empty space one must leave between the rows of gourds is sometimes 16 Amos, sometimes 12 Amos and sometimes 8 Amos...

If all of the work access area is made towards the inside of the field, 16 Amos are required. If it's partially toward the inside and partially towards the outside, 12 Amos are required; if all of it is towards the outside, 8 Amos are required.

(Shmuel): In all cases, 12 Amos are required.